The present invention relates to telecommunications systems and, in particular, to a method and system for reducing power consumption in communications modems that utilize multitone signaling techniques.
The increased use of Digital Subscriber Line (xDSL) technology occurring over the past several years is expected to continue as higher speed and more robust telecommunication connections for long distance multimedia applications (e.g., Internet) are required. Oftentimes in applications such as the Internet, a bandwidth bottleneck is encountered when a two-wire copper twisted pair connection is utilized as the communications medium between user data communications equipment (e.g., a personal computer based modem) and central office data communications equipment. The bottleneck occurs due to the bandwidth constricting nature of two-wire copper twisted pair media as compared to the bandwidth capabilities of fiber-optic media which comprises the backbone of high speed/high bandwidth infrastructures such as the Internet. In the past, the only way to effectively and significantly increase the data transfer rate between user data communications equipment and central office communications equipment was to install a fiber-optic connection between the equipment at both ends.
xDSL technology, including Asymmetric Digital Subscriber Line (ADSL) techniques, has greatly increased the data transfer rate capabilities of existing two-wire copper twisted pair connections. ADSL modems utilize digital signal processing methods and algorithms which utilize a multitone signaling technique known as discrete multitone signaling (DMT), a variant of frequency multitone signaling (FMT).
DMT signals have a relatively large dynamic range, e.g., a peak signal may have an amplitude that is seven times the RMS value of the DMT signal. In other words, DMT signals produce large peaks relative to the overall signal and produce these peaks relatively infrequently (e.g., 5 peaks per second). For typical ADSL down-stream communications (i.e., from central office to end-user), the amplitude of the peak signals is around +/xe2x88x9220 volts while the RMS of the DMT signal is around +3 volts. Nevertheless, regardless of the infrequency of their occurrence, these large peaks must be transmitted and received accurately in order to avoid signal distortion. Consequently, ADSL modems must continuously drive DMT signals at relatively high power levels in order to transmit the signal peaks so that the entire signal can be faithfully reproduced at a receiving ADSL modem.
Accordingly, line drivers of typical ADSL modems utilize power inefficiently because they must continually drive the DMT signal at high power even though only the signal peaks need to be driven as such. While the inefficiencies incurred as a result of driving ADSL signals at high power levels do not generally present a problem for an ADSL modem end-user (e.g., an ADSL modem mounted in a home personal computer) who must power only a single modem, regional telephone companies (e.g., RBOCS) and other telecommunications providers must drive multiple ADSL modems (one for each customer) at their central offices.
As an example, a typical central office may provide service to 1000 ADSL subscribers. Consequently, the power consumed by a system driving 1000 ADSL modems can be as high as 2 k Watts.
Various prior art methods and systems have been introduced to decrease power consumption of modems driving signals with high dynamic ranges. These methods are generally divided between analog-based and digital signal processor-based techniques.
Analog-based power reducing techniques include, for example, the use of output impedance synthesis as described by Victor Koren of Orckit Communications, 38 Nahalat Yitzhak Street, Tel Aviv, Israel 67448, in the Jan. 6, 1994 edition of EDN Magazine in an article entitled xe2x80x9cLine driver economically synthesizes impedancexe2x80x9d which is hereby incorporated herein by reference. Such analog based methods generally utilize signal analysis information obtained via a feedback loop containing various analog circuit components (e.g., resistors) prior to transmission of the ADSL signal. As a result, a significant portion of the power consumed by an ADSL modem using such an analog-based power reducing technique is dissipated in the feedback loop.
Digital signal processor-based power reducing techniques include, for example, a method for adding correcting pulses to a transmitted ADSL signal as described in U.S. Pat. No. 5,835,536 of May et al., entitled xe2x80x9cMethod and apparatus for reducing peak-to-average requirements in multitone communications circuitsxe2x80x9d which is hereby incorporated herein by reference. While generally operating more efficiently in terms of power consumption than analog-based methods of the type described above, digital signal processor based methods have the disadvantage of generally requiring that the digital signal processor algorithm operating in a receiving modem be modified in accordance with signal modifications applied at the transmitting modem.
What is desired, therefore, is a system and method that reduces the power consumption of an ADSL modem without consuming additional power and without requiring the receiving modem to be modified in any way.
The present invention provides a method for reducing power consumption of a modem driving signals having a large dynamic range, e.g., xDSL modems. The preferred method presented includes the steps of: (1) delaying the signal to be driven; (2) determining whether the signal has reached a threshold value; (3) increasing power supplied to a line driver of the modem in response to an indication that the threshold has been reached; and (4) delivering the delayed signal to the line driver.
The present invention also provides a system for reducing power consumption of a modem driving signals having a large dynamic range, wherein the system includes: a digital signal processor, a line driver, a digital delay line disposed between the digital signal processor and the line driver and a threshold detector disposed between the digital signal processor and a power save analog circuit. The power save analog circuit is connected to the line driver and provides a variable power source to the line driver depending upon the state of the threshold detector. The line driver is connected to a transformer which increases the voltage level of the signal transmitted on the DSL communications media. The digital delay line and the threshold detector may be implemented using the digital signal processor. In this way, board space is conserved and known elements and methods for constructing xDSL modems may be utilized.